trial balloon vs balloon

trial balloon and balloon both are nouns.

Word	Noun	Adjective	Verb	Adverb
trial balloon	Yes	No	No	No
balloon	Yes	No	Yes	No

As nouns, balloon is a hypernym of trial balloon; that is, balloon is a word with a broader meaning than trial balloon:

trial balloon (noun)	balloon (noun)
a balloon sent up to test air currents	large tough nonrigid bag filled with gas or heated air
a test of public opinion	small thin inflatable rubber bag with narrow neck

trial balloon (verb)	balloon (verb)
	become inflated
	ride in a hot-air balloon